1. Field of the Invention
The present invention is in the field of pigments particularly useful as flatting agents in low gloss or low sheen applications, such as low sheen paint formulations, and more particularly for weathering-resistant, low sheen paint formulations, and such as low gloss paper applications. The present invention provides pigments and methods for making them using large particle size clays.
2. Background
Kaolin clay pigments are obtained from kaolin crudes. The crudes can contain kaolin particles, oversize particle size impurities (grit), and fine and coarse particle size impurities, such as fine ferruginous or titaniferous impurities, which impart undesirable color to the clay. The kaolin particles typically occur in the crude over a wide range of particle sizes and shapes. Typically a kaolin crude, such as a typical Georgia crude will contain, after removing the grit, particles ranging in size from submicron or colloidal to 20 microns or larger. Typically the kaolin morphology includes arrangement in plates, which plates can be further arrayed in stacks. Particle size, impurity content and morphology can vary with location of the deposit and within any given deposit itself.
Particle size of kaolins are typically determined by sedimentation using Stokes law to convert settling rates to particle size distribution and assuming a spherical shape of the particles. Measurements of particle size determined by this method therefore are termed as e.s.d., i.e. equivalent spherical diameter, as determined by an instrument such as a Sedigraph 5100 (Micromeritics). Typically, kaolin particles finer than about 1 to 2 microns are composed of individual platelets, and particles larger than about 2 microns are composed of stacks or booklets of finer particles.
Calcined kaolins are typically produced by heating kaolins to temperatures generally above 800.degree. C. to 1000.degree. C. This calcination results in the dehydroxylation of the kaolin and transformation into an amorphous aluminum silicate. Typically, calcined kaolins are produced in naturally gas-fired kilns operating either horizontally or vertically. See, e.g., U.S. Pat. No. 3,586,523 (Fanselow et al).
Traditional flatting agents in the paint industry have included diatomaceous earths, coarse calcium carbonates, crystalline silica. Additionally, some calcined kaolins have been utilized as flatting agents, but with limited success, since the paints made with these calcined clays have tended to chalk or discolor during weathering. Traditional opacifying agents include titanium dioxide, which has a relatively high refractive index. Flatting agents are typically utilized in low sheen exterior and interior flat wall paints, and opacifying agents are typically utilized in most paints. In some matte paper applications, flatting agents have also found utility in producing coated paper of controlled lower gloss.
The present invention provides a composition comprising large particle size, calcined kaolin clays that, when used in paper applications, can provide quality low gloss papers, and, when used in paint formulations, provides paints having surprising characteristics: not only low sheen and good opacity, but also good weathering characteristics, i.e., low color change and low change in sheen associated with chalking over time upon exposure. Paints formulated with this inventive product demonstrate high resistance to UV and accelerated weathering, thus demonstrating superior exterior performance. Paints formulated using the present composition can be used both as exterior paints and as interior paints.